This invention relates to a liquid crystal device with a smectic chiral liquid crystal.
Many types of thin display devices have been proposed to substitute for CRT, such as those equipped with liquid crystal or electrochromic material. Particularly, liquid crystal devices are interests of the researchers for their small electric power consumption.
In recent years, however, the number of pixels on a display is now increasing, while the information to be displayed is increasing. Although high quality can be accomplished in the case of a liquid crystal displays having relatively small number of pixels, it is difficult to maintain such a quality when liquid crystal devices are manufactured with a large number of pixels, e.g., into a matrix structure of 640.times.400. In a display having a high resolution, adjacent pixels tend to cross-talk and degrade the overall performance of the display. Several attempt to construct a liquid crystal display with a many number of dense pixels have been conceived. For example, proposed is making use of a ferroelectric liquid crystal as a liquid crystal for such a display, a SBE mode with a conventional twisted nematic liquid crystal as a driving method for the liquid crystal display, a semiconductor device for each pixel, or the like.
There are some shortcomings in manufacturing TN (Twisted Nematic) active matrix display. In the case, the production costs of semiconductor devices are substantially high, the production yield of the same can not be expected so high, and therefore, while the picture quality of the display is rather high, it is very difficult to maintain the price of liquid crystal display proper at a suitable low cost. Furthermore, the response speed of such a conventional device is not so high so that it can not be used for applications which requires quick response. Meanswhile, N.A. Clark et al proposed a new liquid crystal device (Japanease Patent Published Application No.sho56-107216). In the new device are formed a plurality of smectic liquid crystal layers disposed between and perpendicular to a pair of substrates where liquid crystal molecules can be aligned on first and second positions angularly distant from the normal of the layer by .theta. or -.theta. respectively as illustrated in FIG. 1. By applying an electric field to the liquid crystal, the liquid crystal molecules can be switched from one position to the other position, allowing displaying in terms of birefringence.
The molecule can be moved from the first position I to the second position II, e.g., by applying a positive electric field normal to the liquid crystal layer, while it can return from the second position II to the first position when applied a negative electric field. Namely, by changing the direction or sense of electric field applied perpendicular to the liquid crystal layer, Furthermore, the liquid crystal molecules keep their position even if the applied electric field is removed, allowing a bistability between the first and second positions.
An example of a switching signal for such a device is illustrated in FIG. 2. As in the figure, the signal consists of a sequence of positive and negative pulses. The switching of the device can be implemented at a speed higher than that attained by a TN liquid crystal device. When the signal input is interrupted, the liquid crystal molecules remain in the positions as they here.
However, in order to establish the bistability, there is an indispensible requirement that a pair of substrates is provided with an interval (inner) space holding a liquid crystal layer of a thickness of only 1-3 microns inbetween, in order to distort and unwind the spirals of the liquid crystal molecules throughout the layer in the normal direction by applying an electric field and to keep this molecular alignment even when the application is removed. It is very difficult to control such a narrow interval between the substrates, and therefore the yield of mass-production tends to be lowered.